Fuel ignition systems, particularly fuel ignition systems in which hot surface igniters such as silicon carbide igniters are used, are becoming common as an energy saving measure. In many fuel burning systems in the past, a standing pilot which continuously consumed fuel was normally available as the ignition means for the main burner in the system. Since the advent of the fuel energy shortage, many ways have been considered to eliminate unnecessary consumption of fuel. The standing pilot, while very useful in some applications and areas of the country, is wasteful in other applications and areas of the country. As a result of this, efforts are being made to eliminate the standing pilot in as many applications as possible.
Various types of replacements for the standing pilot have been suggested and vary from interrupted spark ignition type devices of an electronic nature to hot surface fuel igniters, such as glow wires or other resistance type elements which become incandescent when electric current is passed through them. In prior art hot surface igniters of the negative temperature coefficient type, one type of failure that can be undesirable is an inadvertent short circuiting of the element itself. This then appears to be an igniter which is functioning properly and some means must be provided to detect this type of unsafe failure. A second type of failure that can be disastrous to a negative temperature coefficient type of hot surface igniter is driving the igniter to a temperature high enough to become destructive to the igniter itself. In this case again, the resistance of the igniter element drops to a very low value and is difficult to separate from an element which is either partially or wholly short circuited by some other type of failure.
Prior art devices have recognized that negative temperature coefficient hot surface igniters such as silicon carbide igniters, can be operated directly in series with a fuel flow control valve. When a source of electric power is applied to this configuration the negative temperature coefficient characteristic of the warning igniter allows the igniter to decrease in resistance value resulting in an increasing current which upon reaching a certain level opens the series connected valve. The valve and the igniter ideally are matched so that the valve opens when the igniter surface is sufficiently hot to ignite the fuel that is supplied upon opening the valve. Either of the above mentioned faults, that is a short circuit of the element, or an overtemperature of the igniter element, are neither detected nor prevented by such a simple arrangement and can be either dangerous or cause a costly failure of the ignition system.